Images collected by a variety of spacecraft, along with elevation data, show conspicuous lines of debris in valleys, almost identical to those seen on top of glaciers on Earth, says Head, a geologist and Mars researcher.

But on Mars, instead of being washed away when the glaciers melted, the lines of debris were deposited intact on the valley floor.

That happened because in the thin Martian air the ice probably went straight from a solid to a gaseous state, a change called sublimation, without any intervening liquid phase.

Climate shifts

What's more, the new evidence of glaciers fits in well with recent models of Mars' climate and new calculations of the gradual, long-term wobble in the tilt of Mars' axis.

Over tens of millions of years, the wobbling axis creates periods when Mars' poles get a great deal more sunshine than today.

Warmer poles would force polar ice to sublime, thickening Mars' atmosphere so the sublimed water could fall as snow at lower latitudes.

"We've been seeing more and more evidence of major climate changes," says Head.

Such evidence has been resisted by people who characterise Mars as dead and dry, says Mars researcher Professor Vic Baker of the University of Arizona.

For about three decades many Mars researchers have been reluctant to accept the idea that Mars might be capable of periodic, fleeting wetter periods, he says. "Now that's not the case."

Volcanos?

Baker says volcanoes might also be warming Mars up periodically.

Just like on Earth, volcanoes on Mars can release large amounts of climate-changing gases and water, he says.

Evidence that the glacial debris comes from one of these brief warming periods, that is, not from Mars' only noncontroversial early wet period 3.8 billion years ago, is the noticeable dearth of impact craters in the glaciated areas, says Head.

What craters are found in the glacial debris offer yet another revelation: they are conspicuously degraded, he says, as if the material ejected from the crater was largely water that has since sublimed away.

Does this mean glacial ice, perhaps just tens of millions of years old, is still there under the surface?

The possibility of buried ice becomes even more plausible in light of discoveries in Antarctica of million-year-old glacial ice buried under a few metres of protective glacial debris, says Baker.

"The [glacial] debris is so thick [on Mars] that it's possible that some of the ice might still be there," says Head.